The present invention relates to a method for producing a crack resistant weld and the fabrication of x-ray devices comprising same. More specifically, the present invention relates to a welding method for joining dissimilar, metallic alloys as well as a fortification metal to produce a weld that is free of cracks.
In an x-ray tube device with a rotating anode, the target consists of a disk composed of a low expansivity refractory metal, for example, a molybdenum based alloy. The x-rays are generated by focusing a narrow beam of high energy electrons onto the target while the target is rotated at high speeds. The target rotates on a center shaft assembly supported by ball bearings. The bearing shaft is typically fabricated from a high hardness tool steel that is able to retain high hardness when exposed to elevated temperatures over extended times. Due to the extremely high temperatures associated with an operating target, the tool steel bearing shaft cannot be attached directly to the target. The connection is established by way of a low thermal expansivity iron-based superalloy mounting hub that joins the target to the tool steel bearing shaft. The superalloy retains high strength when exposed to the high temperatures associated with the joint. The low expansion coefficient of the superalloy material matches closely with the expansivity of the refractory metal target, thus minimizing the contact stresses generated by expansivity mismatch. The integrity of the mechanical joint between the target and the mounting hub must be maintained throughout service, since plastic yielding at the mating surfaces between the two will ultimately result in rotational imbalance and possible premature tube failure.
The iron-based superalloy hub and the tool steel bearing shaft, when joined together, comprise the shaft assembly. Joining these dissimilar materials by a conventional welding method such as Tungsten Inert Gas (TIG) welding significantly simplifies the design of the assembly. For example, welding eliminates the need for either bolted joints or intermediate weld flanges. This is especially important when the design space allocated to the shaft assembly is restricted, which is almost always the case for a rotating anode X-ray tube where efficient use of space is important. Welding, unlike furnace brazing, also preserves the hardened tempers of each of the two materials, and limits annealing to a small heat zone that is affected adjacent to the weld centerline.
Attempts to TIG weld highly alloyed iron-based, low expansivity, gamma-prime strengthened superalloys to high carbon content ( greater than 1.0% by wt.) powder metallurgical, cobalt free, tool steels results in center-line weld cracks even when pre- and post welding heat-treatments are emnployed in the welding procedure. Thus, there remains a need to develop a welding technique that enables the joining of dissimilar, highly alloyed metals to form a stable weld joint without cracks at high temperatures.
The present invention is directed to a welding method that enables the joining of at least two dissimilar, metallic alloys without the formation of cracks. More specifically, the present invention is directed to a welding method that allows the joining of a highly alloyed iron-based, low expansivity, gamma-prime strengthened superalloy (for example, Pyromet Alloy CTX-909, Incoloy Alloy 903, Alloy 907 and Alloy 909) to a high carbon, powder metallurgical, cobalt free tool steel (i.e., CPM REX 20) that is high in refractory metal alloying agents. Cobalt free tool steels with chemical compositions similar to CPM REX 20, that contain a high weight % of carbon ( greater than 1%) and total refractory metal additions greater than 15 wt. %, are likely candidates for use in the present invention.
By incorporating commercially pure nickel (99.00% minimum by wt.) of sufficient quantity between the two alloyed metals to be joined and incorporating the nickel into the weld, a weld free of both centerline and tail cracks is obtained. The weld geometry is also designed to expose a portion of the nickel wire surface to the weld flame and, thus, enhance the mixing kinetics and resultant alloying of the nickel into the weld. The nickel inoculant can be introduced either as a formed wire or pre-formed washer. Although the details of the present invention focus on the use of a nickel wire, it is not intended that the present invention be limited as such. Further, the addition of nickel enables the weld to be constructed using standard TIG welding methods.
The benefits that are achieved with the welding technique of the present invention are best illustrated in the fabrication of a rotating anode bearing shaft assembly of the type that is used in an x-ray tube device and disclosed herein below. The bearing shaft assembly comprises an iron-based superalloy hub and a tool steel bearing shaft. The integrity of the joint that connects the superalloy hub and the tool steel bearing shaft must be maintained throughout the operation of the x-ray tube, otherwise rotational imbalance and premature tube failure will likely occur. The superior weld joint that is achieved with the method of the present invention overcomes the disadvantages observed with existing joints such as weld centerline cracks. By providing a weld joint of the type used in the present invention to join together dissimilar, highly alloyed metals under high temperature and stress conditions, articles of manufacture are produced having improved and unique thermo-mechanical properties. For example, the rotating anode bearing shaft assembly of the present invention exhibits superior properties, which include:
1. ease of assembly;
2. a tool steel bearing shaft free of the cost considerations associated with cobalt;
3. the retention of the full-hardness, high strength temper in both the bearing shaft and the hub;
4. a high strength hub that possesses a low coefficient of thermal expansion (CTE); and
5. economy of design space by avoiding intermittent weld flanges.
The method of the present invention provides a welding method, the placement of a nickel fill wire in the joint to be welded as well as the chemical characteristics of the weld both with and without the nickel fill wire. Accordingly, it is an objective of the present invention to provide a bearing shaft assembly possessing unique thermo-mechanical properties. It is a further objective of the present invention to eliminate the need to have intermediate welding flanges or couplings that would otherwise be necessary, thereby minimizing the allocated design space for the bearing shaft assembly within the X-ray tube.